1. Field of the Invention
The present invention relates to a wide-angle optical system for a solid image pickup element, and particularly, to a wide-angle optical system for a solid image pickup element, which is suitable for use in an image pickup device utilizing an image pickup element mounted in a mobile telephone, a PC camera and the like.
2. Description of the Related Art
In recent years, the demand for a camera utilizing an image pickup element such as CCD, CMOS and the like adapted to be mounted in a portable computer, a visual telephone, a mobile telephone and the like has being increased with the development of multimedia.
It is desired that such a camera is small-sized and lightweight, because the camera is required to be mounted in a limited space.
Therefore, it is desired that an optical system used in such a camera is likewise small-sized and lightweight.
There are conventionally known solid image pickup elements which are suitable mounted in a mobile telephone, and which include a type having a high resolution of about 300,000 picture elements, which is called a so-called VGA, and a type having a resolution of about 110,000 picture elements, which is called a so-called CIF.
In an optical system used in such a camera, two or three lenses may be used when a higher performance is desired. However, to realize a reduction in size and a reduction in cost, a single-lens optical system is employed in many cases. Such a single-lens optical system conventionally employed includes a type in which a single image pickup lens such as a biconvex lens and a meniscus lens is incorporated.
Such conventionally known single-lens optical systems are known from Japanese Patent Application Laid-open Nos. 06-88939 and 11-264930 in which a biconvex lens is employed, and from Japanese Patent Application Laid-open No. 10-282410 in which a meniscus lens is employed.
However, such conventionally known single-lens optical system suffers from a disadvantage that although a performance sufficient to a certain degree is obtained in the viewpoint of the resolution, a large distortion remains.
The distortion offers a less problem in a case of a small displaying surface, but is more conspicuous as the displaying surface is larger. For this reason, the distortion is one of subjects which are to be required to be improved immediately.
Further, there are conventionally known single-lens optical systems described in Japanese Patent Application Laid-open Nos. 61-16314, 06-118300. Any of these single-lens optical systems is not an optical system for picking up an image, and is an optical system proposed as being used in various sensors or for photometry and thus, is different largely in respect of application from that according to the present invention. It is the actual circumstance that the angle of view is smaller (e.g., the maximum angle of view is on the order of 35°), and the solution of the above-described problem in the wide-angle image pickup optical system is not proposed at all.